Write proposal draft for directory mirrors to accept, aggregate and hand off descriptors to dirauths

In the event of DoS or braindead client behavior, directory authorities may need to rate limit or restrict connections. See #2665 (moved).

Under these conditions, it would be useful if directory mirrors could also accept relay descriptor data, aggregate it, and hand it off to the authorities after eliminating duplicates. This coupled with #572 (moved) should allow the dirauths to better handle sudden traffic spikes by rate limiting or firewalling, without degrading the network.

https://gitweb.torproject.org/torspec.git/blob/HEAD:/proposals/147-prevoting-opinions.txt has a related idea, but we may want a push method rather than a pull?